U.S. Patent Application Publication No. U.S. 2013/0345646 A1 (Bertrand et al.) describes an implantable physiological shunt system which utilizes a magnetic coupling between an external adjustment tool and an internal magnetic rotor assembly in order to control the flow of fluids. The shunt system can include a locking feature to prevent unintended setting changes when the shunt is exposed to strong external magnetic fields.
This feature can be particularly important when shunts used to control the flow of cerebrospinal fluid (CSF) from the brain ventricles of hydrocephalus patients are exposed to external magnetic fields during magnetic resonance imaging (MRI). However, if the shunt lock is set, the internal magnet may also be prevented from aligning with the external magnetic field, and in a sufficiently strong external magnetic field may become demagnetized or reverse magnetized. If this occurs, surgical replacement of the shunt may be required.
External magnetic fields of for example as high as 3 Tesla are generated in some MRI scanners. Fields of that strength can demagnetize or reverse magnetize samarium cobalt (SmCo) magnetic materials. NdFeB (Neodymium) rare earth permanent magnets have sufficiently high coercivity (Hci) to resist demagnetization or reverse magnetization in such fields, but also have very poor corrosion resistance. Magnetic strength generally is lost in direct proportion to the mass loss caused by corrosion. NdFeB magnets typically are made more resistant to corrosion by applying protective coatings such as plating (e.g., nickel plating or layers of copper and nickel plating), powder coatings or paints. However, when such protectively coated NdFeB magnets are submersed in aqueous saline solution, the protective coating may in some cases be breached within 24 hours after the start of exposure.